U.S. patent number 4,995,083 [Application Number 07/387,156] was granted by the patent office on 1991-02-19 for method of establishing a radio link in one of a plurality of channels between two radio units.
This patent grant is currently assigned to GEC Plessey Telecommunications Limited. Invention is credited to John M. Baker, Andrew S. Pearce, Peter N. Proctor.
United States Patent |
4,995,083 |
Baker , et al. |
February 19, 1991 |
Method of establishing a radio link in one of a plurality of
channels between two radio units
Abstract
The method comprises transmitting an initiating signal in the
form of a bit pattern as a continuous burst from one radio unit to
the other. The pattern incorporates a preamble pattern and a
security code having a plurality of bit sequences. Each sequence is
transmitted a number of times. The received security code is
compared with stored codes in the other unit, and if a matching
pair is identified a response signal is transmitted from the other
unit to said one unit. The response signal incorporates the
preamble pattern for establishing synchronization between the radio
units.
Inventors: |
Baker; John M. (Basingstoke,
GB3), Proctor; Peter N. (Basingstoke, GB3),
Pearce; Andrew S. (Nottingham, GB3) |
Assignee: |
GEC Plessey Telecommunications
Limited (Coventry, GB2)
|
Family
ID: |
10643075 |
Appl.
No.: |
07/387,156 |
Filed: |
July 31, 1989 |
Foreign Application Priority Data
Current U.S.
Class: |
375/365; 375/377;
380/274; 455/410 |
Current CPC
Class: |
H04M
1/727 (20130101) |
Current International
Class: |
H04M
1/727 (20060101); H04Q 7/38 (20060101); H04M
1/72 (20060101); H04K 001/00 (); H04K 009/00 ();
H04M 011/00 () |
Field of
Search: |
;379/62 ;380/23
;455/33,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Buczinski; Stephen C.
Attorney, Agent or Firm: Fleit, Jacobson, Cohn, Price,
Holman & Stern
Claims
We claim:
1. A method of establishing synchronization in a digital radio link
in one of a plurality of channels between two radio units of a
multi-channel communication system, comprising transmitting an
initiating signal in the form of a bit pattern as a continuous
burst from one radio unit to the other unit, said continuous data
stream burst having a timing which is independent of the
synchronism achieved, and said other unit temporarily achieving bit
synchronism without changing frame synchronism, the bit pattern
incorporating a preamble pattern and a security code comprising a
plurality of bit sequences, each bit sequence being transmitted a
plurality of times in said continuous burst, comparing the received
security code with one or more codes stored within said other radio
unit, whereupon, if a matching pair of codes is identified, a
response signal is transmitted from said other radio unit to said
one unit, the response signal incorporating the preamble pattern
for establishing synchronisation between the radio units.
2. A method of establishing synchronism in a digital radio link as
claimed in claim 1, wherein one radio unity is a portable unit and
the other is a fixed unit and together form a cordless
telephone.
3. A method of establishing synchronism in a digital radio link as
claimed in claim 2, wherein the portable unit acts as a synchronous
slave to the fixed unit, taking timing of transmit and receive
frames from the fixed unit.
Description
FIELD OF THE INVENTION
The present invention relates to a method of establishing a radio
link in one of a plurality of channels between two radio units of a
multi-channel communication system.
The present invention finds application in the field of mobile
telecommunications systems and particularly cellular radio
telecommunications systems.
SUMMARY OF THE INVENTION
The present invention has the advantages of reducing the cost of
multiple radio link base stations used in such systems and permits
production of identical handsets for domestic and business use.
STATEMENT OF THE OBJECT OF THE INVENTION
According to the present invention there is provided, a method of
establishing a digital radio link in one of a plurality of channels
between two radio units of a multi-channel communication system,
comprising transmitting an initiating signal in the form of a bit
pattern as a continuous data stream burst from one radio unit to
the other, the bit pattern incorporating a preamble pattern and a
security code comprising a plurality of bit sequences, each bit
sequence being transmitted a plurality of times in said continuous
data stream burst, comparing the received security code with one or
more codes stored within said other radio unit whereupon, if a
matching pair of codes is identified, a response signal is
transmitted from said other radio unit to said one unit, the
response signal incorporating the preamble pattern for establishing
synchronisation between the radio units.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be described with
reference to the accompanying drawings wherein:
FIG. 1 shows the format of a handshake signal,
FIG. 2 shows the format of a call control signal and,
FIG. 3 shows a link level sequence diagram.
In a digital cordless telephone system utilizing multiple radio
channels and achieving duplex operation by time division duplex,
digital bit and frame synchronisation has to be established before
communication can proceed.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The radio system comprises cordless portable units and cordless
fixed units where the cordless fixed units may comprise either a
single radio subsystem utilising a single radio channel at any one
time, or multiple radio subsystems utilising multiple radio
channels simultaneously.
In order to establish synchronism, the portable unit acts as a
synchronous slave to the fixed unit taking the timing of transmit
and receive frames from the fixed synchronism of the fixed unit.
This ensures that all radio subsystems in a multi-channel fixed
unit will remain in fixed transmit and receive synchronism, one
with every other, independently of the timing starting or ending
link establishment.
When the cordless portable unit initiates the connection a burst
comprising a continuous data stream of 25 frames is transmitted
without pause. The data stream contains a repeated sequence of a
unique data marker code preceding the identity of the portable
unit. The sequence is such that the fixed unit is able to
temporarily achieve bit synchronism without changing frame
synchronism and by reading every other frame (i.e., fixed part
receive frames) is able to extract the handset identity regardless
of where the boundaries in the 25 frame transmission lies with
respect to the fixed unit frame synchronism.
At the end of the continuous transmission the portable unit goes to
a continuous listen mode. The fixed unit commences burst mode
transmission of a link synchronisation control signal to its fixed
frame and bit synchronism.
The portable unit on receiving the link synchronisation control
signal now establishes a burst frame and bit synchronism derived
from the incoming fixed part signal, and commences to transmit
burst synchronised to the receive period of the fixed part
unit.
The link is now established and communication of telephony
signalling and voice can proceed.
To establish a radio link two basic types of control signal are
used. The first, shown in FIG. 1 and is a handshake type of signal
which carries the security code, 1 for identifying fixed and
portable units to their associated counterparts but carries no
error detection field (CRC field). This type of signal takes two
forms the regular `Handshake` signal which is used to verify
association between units during a call. This form is signified by
a `zero` in the Handshake Type Field, 2 and the `Clear Handshake`
signal which is used to clear down connections. This form is
signified by a `one` in the Handshake Type Field, 2. At least three
consecutive valid "Clear Handshake" signals must be received by a
part before it clears down in response.
The second type of signal consists of the call control set of
signals as shown in FIG. 2. This set carries no security code but
possesses an error detection field, 3. The acknowledge signal is
included in the Call Control signal set.
The constituent parts of the call signals are as follows:
Leading Flag (Header)
This field 4, indicates the start of a level 2 signal frame to the
receiving station. It is an 8-bit field and takes the binary
form:
A trailing flag is unnecessary with this form of compelled
signalling.
Bit Stuffing
For all fields other than the Leading Flag a logical `zero` is
inserted after any sequence of five logical `ones` is encountered.
This is applied even if the sequence of five `ones` crosses field
boundaries.
The Leading Flag is therefore uniquely identified as a sequence of
size logical `ones` bounded on each side by a logical `zero`.
The Bit Stuffing operation, is carried out in the burst mode
device.
Control Signal Function Field
The function field 5 consists of 8 bits. The function field
contains the information that is to be transferred e.g. dialled
digits etc.
Acknowledge Signal
A special signal which can be included in the set of Call Control
signals is the Acknowledge signal. This signal would of course only
be sent in response to an incoming Call-Control signal frame and as
such, in a compelled signalling scheme as outlined, would not enter
into the signal sequence numbering scheme. There are two principal
reasons for this:
(a) There is no need to number the Acknowledge signal because the
link end which transmitted the original Call Control signal will
keep re-transmitting it until it receives acknowledgement.
(b) At a given link end, acknowledgements of received signals can
be interspersed with signals originating at that end. In this case
the integrity of the sequence numbering must be preserved for the
originating signal sequence.
Acknowledge signals should therefore be unnumbered and the modulo 2
numbering sequence should only be updated at a link end by Call
Control signals, other than acknowledgements, originating at that
end.
The specific format for the (unnumbered) Acknowledge signal can be
a Call Control signal with all logical zeros in the Function field.
When this signal is transmitted, no updating of the Signal Sequence
Number takes place and when this signal is received, no account is
taken of the Signal Sequence Number field.
Security Code
The final field 1, in the handshake signal frame, is the security
code. This field is 22 bits in length resulting in 4,194,304
possible combinations which are used to uniquely associate the base
station (fixed unit) with the handset, (portable unit). This code,
must be read directly from the associated memory device.
Handshake Clear
Upon receipt at the opposite parts of a handshake clear signal, the
respective parts enters the `clear down` sequence. This is
indicated by a `1` in the handshake identifier field, 6.
Error Detection
The method for providing error protection for control signal
messages transmitted across the signalling link consists of a
scheme where signals are checked for errors at the receiver. If no
errors are found, an acknowledgement is returned. Conversely, if an
error is found, no acknowledgement is returned, in which case the
sending end re-transmits the signal.
A cyclic redundancy check (CRC) field 3 is appended to the control
signal to enable error detection to be carried out at the
receiver.
The generator polynomial which results in a CRC field of 6 is given
below. This generator polynomial results in the following
characteristics:
(a) The number of bits in the CRC field (CRC BITS)=6.
(b) The maximum number of bits in the information field (INF
BITS)=25.
(c) Error detection capabilities guaranteed by using the
polynomial, provided the stated maximum number of bits in the
information field is not exceeded, results in two bursts of length
2 or less. Any odd number of errors. A burst of length 6 or less
96.88% or burst of length to 98.44% of longer length.
The generator polynomial from which the error correction
capabilities of the codes are derived is:
To establish a radio link the handshake signal is sent from the
calling subscriber under the control of a timer. The signal is
subjected to a timeout indicating loss of handshake signals if a
link is not established. When the link is established the handshake
signal from the called subscriber is sent with matching security
code. The handshake signal is subject to timeout. The link is
cleared down when the timeouts have expired or when the call
originator clears down.
The protocol for sending call control signals is as follows. Data
is put into the function field, and other call control signal
fields are attached. The handshake identifier bit 6 is made equal
to zero, and the current sequence number is put into the signal
sequence number field, 7.
The cyclic redundant check code value is complied and put into the
CRC field, 3.
The call control signal is sent to the other unit, and an
acknowledgement timer is activated. The call control signal is
repeatedly sent until the acknowledgement signal is received,
whereupon the signal sequence number is incremented.
Referring to FIG. 3, the interchange is shown between an
originating radio unit ORU, and the receiving radio unit, RRU. The
RF link is shown between the two radio units. The originating unit
indicates a call CI and finds a free channel FCF. The security code
SC is sent continuously. Meanwhile the receiving unit is idle and
scans the channels, ISC. When the receiving unit receives the
handshake HSK it returns a security code SC to the originating unit
on a continued basis. The originating unit receives a returned
handshake RSHK, and seizes SZE, the RF link. If an acknowledgement
timer ACK in the originating unit expires seizure SZE continues as
required. The receiving unit recognises the seized condition and
the link is established, L EST. An acknowledge signal ACK RT is
returned to the originating unit when further seizure signals a
received by the receiving unit. When the acknowledge signal is
received by the originating unit it recognises the establishment
L.EST of the RF link.
* * * * *